After you file

Sorry you can't update them.  The 2020 return only transfers in when you start a new blank 2021 return so you would have to start  over to do that.   Some items you can update like carryovers.  What do you want to change?  They might not affect your 2021 return.    Nothing you do in 2020 will update 2021 unless you start a new 2021 return and  Transfer again.